Tracking filters are extensively used for tracking signals in communication systems where unwanted signals and the ambient noise content are high. For example, the signals in space communication systems are subject to wide dynamic changes of frequency and a high additive noise level and the receiver must be able to track the wanted signal embedded in the noise and unwanted signal spectrum. Usually, a transponder on board a space vehicle is provided with a tracking filter to track the high wanted signal from an incoming spectrum of signals having a high noise content and being subjected to a dynamic rate of change due to the Doppler effect, as it approaches and recedes from a ground transmitting station.
The signal used in such an environment is typically phase or frequency modulated and by the time it reaches the receiver, it is a narrow band signal which has undergone a wide shift of frequency due to the Doppler effect and is deeply imbedded in additive noise. For example, a modulated signal having a band width of 20 Hertz may undergo 200 KHertz shift due to the Doppler effect, and is also buried in or accompanied by a high ambient noise level. Heretofore, generally a receiver is provided with a tracking filter designed to track the signal of the aforementioned nature, and to extract the wanted signal. Typically, such a filter includes a phase locked loop having a phase detector, a filter, and an oscillator designed to track and recover the wanted signal and attenuate the unwanted signals and noise from the incoming signal spectrum. However, the attenuation is not complete because attempts to attenuate unwanted signal completely tended to cause the tracking filter to be unable to track the wanted components.